What is FLB?
FLB stands for Floating Lens Block. It is a mechanical movement system designed and developed by Skyllaney Opto-Mechanics for Omnar Lenses. The FLB system ensures focal plane stability while mitigating or counteracting the effective focal length (EFL) recompilation of a lens formula. EFL recompilation occurs when the iris diameter contracts, gradually blocking the lower focal length of the lens’s outer elements, leaving the higher focal length of the central elements dominant.
The FLB system operates by moving one or more lens elements within the optical formula to counteract or mitigate EFL recompilation triggered by changes in the f-stop (light transmission value) due to iris diameter contraction or expansion.
How does FLB differ from FLE?
FLE stands for Floating Lens Element. It is an umbrella term used to describe one or more lens elements in an optical formula that move independently as the lens is focused across varying distance ranges. FLE is typically activated by adjustments to the focus ring on a lens.
In contrast, FLB is activated by adjustments to the aperture ring. While FLE aims to maintain optimal sharpness within a given depth of field zone in relation to the lens’s focal plane, FLB targets focal plane stability across aperture values.
Can FLB and FLE be combined in a single lens design?
Yes. While FLE focuses on achieving maximum sharpness and EFL stability across the focus range, FLE cannot compensate for focus shifts that are aperture induced. FLB on the other hand, ensures focal plane stability across all aperture/ transmission values. Combining both systems allows for a lens design that maintains sharpness and stability throughout its aperture and focus range, resulting in a formula with neither focus breathing or focus shift.
Can FLB be used with Aspherical (ASPH) and Apochromatic (APO) lens designs?
Absolutely. A lens formula, whether ASPH or APO in its per-element configuration, can incorporate an FLB design into its mechanical structure. There is no reason why an ASPH APO FLE FLB lens formula could not be designed. Such a lens would achieve high wide-open MTF performance (ASPH), chromatic aberration elimination (APO), maximum nodal point sharpness across the full focus range (FLE), and zero focus shift (FLB).
Can FLB resolve focus shift in older lens formulas?
Indeed. Classic formulas like the Carl Zeiss Jena 8.5cm f/2 Sonnar and the Leica 75mm f/1.4 Summilux, which suffer from mid-aperture focus shift, could be mechanically redesigned to implement FLB into their optical block carriage systems. By doing so, the rear optical set of these formulas could be manipulated by the aperture control ring to reposition itself along the optical axis, counteracting the EFL recompilation that occurs when the iris blades contract (stopping down to reduce light transmission).
Implementing the FLB system correctly requires understanding how much the EFL value changes from its wide-open transmission rating to its closed-down minimum transmission rating.
FLB Implementation Examples
The Carl Zeiss Jena 5cm f/2 Sonnar is a native 52.4mm EFL-rated lens with varying focal lengths across its elements—centrally higher and peripherally lower.
As the lens is stopped down to f/2.8, the aperture blades gradually cover the outer element areas, making the central focal length more dominant in the lens’s collective EFL averaging. Around f/3.5, the EFL value begins to drift. As the iris constricts further to f/4.5, the EFL value further drifts, and near f/5.6, it reaches its peak EFL recompilation. The exact EFL shift depends on the specimen’s starting EFL, the outer element focal length wide open, and the central axis focal length.
Depending on the individual lens formula, there exists several FLB design implementations that could resolve any potential EFL recompilation issues. A few are elaborated on below:
- FLB Rear Carriage Repositioning (control over only the rear set of the optical formula):
By controlling the rear set of the focus-shifting optical formula via a precision FLB carriage system design that maintains perfect optical concentricity, the rear optical carriage moves in either a positive or negative optical axis direction (formula dependant) at a rate that cancels out the iris contraction’s EFL shift. This achieves focal plane stability via EFL value stabilization, independent of the focal registration distance. - FLB Front Carriage Repositioning (control over only the front set of the optical formula):
This method operates similarly to the rear carriage repositioning but with the front set performing the linear translations induced by aperture value changes. Omnar Lenses has used this FLB system front carriage method on the 35mm Pantessa lens and the forthcoming 28mm Pantriplet lens to correct focus shift in these formulas. - FLB Total Optical Block Repositioning (control over both the front and rear sets of the optical formula):
In this configuration, counteracting focus shift involves moving both the front and rear optical sets. This method is advantageous in complex optical formulas where optical timings (rotational positions of elements) need to be retained to ensure maximum control over unwanted aberrations or de-centering due to varying glass element tolerances. It permits the use of a rotating FLB carriage system if spatial constraints dictate, while maintaining element-to-element optical timing relationships.
The Versatility of the FLB System
The beauty of the FLB system design lies in its flexibility. Whether it involves front set, rear set, single element, or whole formula synchronous or asynchronous movements, the key is that the movements are induced by the aperture control system to counteract or mitigate EFL recompilation. This adaptability allows optical and mechanical design engineers to tailor the FLB system to the specific requirements of the lens formula in question.
Focal plane stability across all aperture transmission values can be achieved by maintaining the lens’s starting wide-open aperture EFL, which includes the focal length in the outer element, at the closed-down minimum aperture setting.
This is accomplished by manipulating the spacings of the front and rear sets so that the central element focal length gradually becomes the starting EFL as it achieves dominance, while the outer element focal length loses its EFL influence.
FLB technology has significant implications in rangefinder photography and cinematography. By addressing focus shift, FLB enables classic lens formulas to perform optimally across all aperture values. This advancement opens up new possibilities for filmmakers and photographers, allowing them to utilise vintage lenses with modern precision.
Implications for Cinema Lenses
One of the major challenges in adapting older lens formulas for modern cinema use is addressing their inherent focus shift. Focus breathing refers to EFL changes across a lenses focus throw, while focus shift refers to EFL changes across a lenses transmission variation. While additional lens elements can be retrofitted to introduce FLE (Floating Lens Element) functionality and improve close-focus sharpness and mitigate focus breathing, this alone does not correct the focus shift that occurs as the iris is contracted. As the lens’s transmission value (T-stop) is adjusted, the shift in effective focal length causes the focal plane to move, throwing focus off the intended subject.
Incorporating FLB (Floating Lens Block) design into existing lens formulas being rehoused for cinema applications should, in theory, eliminate focus shift across the entire T-stop range. As demonstrated in the three FLB system configurations discussed earlier, this can be achieved by dividing the classic lens formula into two optical groups—one of which is mechanically coupled to the aperture control system via the FLB carriage. During T-stop changes, repositioning either the front or rear half of the lens formula along the optical axis enables consistent focal plane alignment. This method has proven effective even with lens designs particularly prone to focus shift, and the same approach applies when recreating classic formulas from scratch.
Without the need to suppress and negate aberrations thru additional element counts to prevent focus shift, FLB opens the door to the creation of non-focus shifting and aberration rich art lenses, quite possibly the likes of which have never been seen before. This has a design impact on the cinema lens industry whom already appreciate the value of classic cinematic rendering but equally have little tolerance for focus breathing or focus shifting while filming. While there is no doubt that FLB will be significant for the rangefinder lens industry, its impacts will be felt even bigger in the cinema lens industry once the full scope of its focal plane stability technology across varying light transmission function becomes more widely understood and integrated into future designs.
Design Challenges
Implementing the FLB system typically results in a larger overall mechanical diameter compared to conventional (non-FLB) lens designs. This is due to the need for additional mechanical components such as couplers, carriages, and internal helicoids required to translate rotational aperture adjustments into linear, stabilised movements along the optical axis.
For instance, in a rear FLB carriage design—such as that used in the Bertele 5cm f/2 FLB lens—the front aperture ring drives the rear optical group via couplers that translate rotational input into precise axial movement. Omnar Lenses has adopted a “helicoid-within-a-helicoid-within-a-helicoid” configuration to implement FLB functionality. In this arrangement:
- The innermost helicoid controls FLB carriage movement, linked directly to the aperture control ring via a mechanical coupler;
- The central helicoid manages rangefinder cam translations;
- The outer helicoid provides the standard focusing mechanism.
The result is a visually intricate, tourbillon-like system of interlocking helicoids—particularly striking when viewed in cross-section along the aperture plane. Naturally, this triple-layer configuration increases the lens’s midsection diameter, as the additional space is needed to accommodate the FLB components and their associated mechanics.
Manufacturing Challenges
The FLB system developed for Omnar Lenses is the result of over five years of research, development, investment and engineering. It provides a purely mechanical solution to an optical design issue that has persisted for more than 90 years:
How can unwanted optical EFL (Effective Focal Length) recompilations—which result in focal plane shifts on manually operated mechanical lenses—be effectively managed?
One of the greatest challenges lay not only in designing an assembly to address this problem, but also in manufacturing it to fit within a minimal footprint while ensuring maximum optical concentricity in the finished lens.
During the prototyping phase, it quickly became apparent that standard machining tolerances of ±0.100 mm (100 µm) were insufficient to achieve a properly functioning FLB helicoid and optical block assembly. Such tolerances cannot centre floating lens elements accurately enough for high-megapixel imaging standards. Furthermore, even finer tolerances of ±0.050 mm (50 µm) proved inadequate in this regard. This will likely be a point of concern for many modern lens manufacturers, as rangefinder lenses are often produced using general tolerances for most parts, and finer tolerances only for select critical components.
Depending on the optical formula, the optical axis shift movements needed to provide EFL stabilization across each aperture value can be quite minute. On the several different lens formulas which contained EFL induced focus shift that Skyllaney has tested FLB designs on, most of the optical axis shifts per aperture / transmission stop movements are merely 10-15 microns. Moving an optical cell precisely these few microns per aperture stop without introducing pitch or yaw shifts is paramount in successfully implementing FLB into a lens formula.
In the context of rangefinder lenses, manufacturing costs rise exponentially with increasingly fine tolerances. Leica Wetzlar in Germany is widely regarded as the benchmark in this field. Value-oriented manufacturers may find it difficult to integrate a working FLB system into their designs without incurring substantial cost increases, given the precision and tolerances required to ensure correct FLB operation without introducing optical de-centring during carriage movement.
In summary, ultra-fine machining tolerances of ±0.005 mm (5 µm) on both the FLB carriage and its corresponding helical threads—which secure the optical groups—are essential to avoid optical de-concentricity. These are the tolerances currently achieved on all Omnar FLB lenses by Skyllaney Opto-Mechanics UK.
Specialised Machinery
To meet these stringent tolerances, specialised manufacturing equipment—unavailable commercially—had to be designed and built entirely in-house. Both the FLB carriage and its helical threads are initially machined to a base specification on high-end modern CNC machines. However, to create precisely matched toleranced pairing of parts, Skyllaney Opto-Mechanics developed two custom ultra-precision CNC machines from scratch: a 4-axis CNC mill and a 3-axis CNC lathe.
These machines utilise multiple measurement feedback systems, including motor-level leadscrew step counters, and both glass and magnetic scale X-Y-Z-A axis readouts. These systems continuously track the tool and material positions using competing methods to enable peer-averaged dimensional verification. Each machine can repeatably position its tool to within 5 µm, ensuring all mating and interlocking FLB components fit together seamlessly. Optical magnifiers built into these machines permit the operator to see a macro view of the part as it is being machined. In some cases, machining of the parts features 95% of the way is done via automation, with the final few passes performed manually until target tolerances are reached.
Additionally, the ability to transfer parts between the 5 µm precision CNC mill and lathe without removing them from their collets or vices proved crucial for maintaining the FLB optical block’s concentricity. Skyllaney Opto-Mechanics developed a standardised work-holding interface across all CNC machines in their facility, allowing parts to be moved between machines without losing alignment with the optical axis. This system minimises runout and maximises accuracy.
Critical parameters—such as vibration, angular steps, tool chatter, thermal variation, tool RPM, and feed rate—are continuously monitored to ensure tolerances are held throughout the machining process. Nearly every FLB lens component has its own dedicated tooling, jigs, and support accessories. Some machines are assigned exclusively to producing features on specific parts, and many components require multiple machining operations across different machines to complete their geometries and dimensions.
Throughout this ultra-precise machining process, not only are digital readout scales regularly checked against the leadscrew step counters for consistency, but components are also continuously verified using Carl Zeiss Jena and Mitutoyo micrometers and calipers, capable of measuring at the required resolutions. The dual DRO feedback systems on the ultra-precision CNC machines alert operators to any developing tolerance drift by monitoring leadscrew positions, tool and cutter positions, and X-Y-Z-A table coordinates in real time, comparing them against the CAD/CAM model to detect any dimensional mismatch between the digital design and the physical part.
While there exists manufacturing challenges in implementing FLB successfully in a camera lens design, the end result is the elimination of focus shift which for certain lens formulas like the Sonnar, bring it back into the modern fold of usability amongst its peers.
Conclusions
FLB offers the opportunity for classic lens formulas to make a remanufactured comeback, no longer held back by their optical design faults which arguably also contributed to their beloved rendering. While the era of FLB lens design is only now starting, it has the potential to upset the established order of complex multi element lens design whose focus often was focal plane stability thru aberration negation which more often then not would lead to bokeh degradation and the general sterilisation of rendering.
This philosophy of managing and suppressing unwanted aberrations thru additional element counts combined with prioritising maximum cross plane modulation transfer function (MTF) as a metric for benchmark against competing offerings for marketing purposes, has lead to many of the large and complex multi-element lens designs today that become indistinguishable from one another.
The advent of artificial intelligence and smartphones can now replicate the look of various modern camera lenses. It is in the imperfection of classic formulas like the Sonnar where a growing number of photographers now find themselves gravitating towards, if for no other reason than to give a sense of purpose, individuality, creativity and character in the form of an artistic brush stroke to the image making process that often feels missing on many modern lens offerings.
Author:
Christopher C. Andreyo – Managing Director, Head Engineer
Skyllaney Opto-Mechanics, Alexandria, Scotland
For Omnar Lenses
30th May 2025
Peer-review by Brian Sweeney
During the development of the Bertele lens over the past five years, we have been in regular contact with Brian Sweeney, who has provided technical engineering feedback on the design and also beta-tested the original non-FLB Bertele prototype.
For those who may be unfamiliar with Brian, he is widely regarded—by both ourselves and many others—as one of the world’s foremost experts on the 5cm f/2 and 5cm f/1.5 Carl Zeiss Jena Sonnar lens formulas. Over the past few decades, Brian has serviced, converted, and recalibrated hundreds of Sonnar lenses. His understanding of the formula—right down to the manufacturing and mechanical engineering level—is both intricate and profound, making his input invaluable throughout the development process.
Much of what Skyllaney initially undertook was built upon the technical foundation laid by Brian’s earlier work. Decades ago, he authored white papers detailing the process of converting Contax-calibrated Sonnars to Leica RF calibration, as well as describing in depth the countless nuances and variations of these lenses produced from the 1930s through to the 1950s. His insight is drawn from extensive first-hand experience working on them.
We invited Brian Sweeney to peer review the FLB technology outlined above.
His comments are presented below:
Background: All lenses “suffer” from focus shift when stopping down the aperture due to spherical aberration. This is a direct result of the focal length of the lens changing as a function of the aperture. Optical engineers have minimized focus shift due to spherical aberration in some optical designs by using aspherical elements and optics with long, drawn-out optical paths. These solutions introduce their own set of drawbacks: increase in size and weight; higher cost and complexity; and displeasing optical effects known as “onion bokeh.”
The Sonnar is an “asymmetric” configuration: the front section is a telephoto design with a focal length about 2.5x the overall focal length of the lens; the rear group is about the same as the final focal length. This gives the Sonnar a compact design. The Sonnar design introduces more spherical aberration than is present in a symmetric-type lens. The focal length of the center of the 5cm Sonnars is longer than the focal length of the edges. This means the best focus point of light entering the Sonnar from the center is behind that of light entering from the edge. Used wide open, the image is dominated by light coming in at the shorter focal length of the edges. This is responsible for the lower-contrast, spread-out depth of field of the Sonnar when used wide open. Stopping down the aperture eliminates contributions from the edge; the image that remains is the product of the longer focal length center of the lens. It “shifts” toward infinity.
Long-time users of the Sonnar on a rangefinder camera learn to “manually” compensate for focus shift. The typical rule of thumb is to either move back a little when using the lens stopped down, or first focus using the rangefinder, then adjust the focus slightly closer.
Skyllaney has invented a technique to correct focus shift due to spherical aberration and has implemented this technique in the new Omnar Bertele 5cm f/2 Sonnar formula lens. The Skyllaney invention couples the aperture adjustment with the optical block of the lens. Focus is maintained throughout the aperture range using mechanical compensation to physically move the entire optical block. Alternatively, the Skyllaney invention can be used to optically compensate focus shift due to spherical aberration by changing the spacing between optical groups to maintain focal length as the aperture is changed.
Mechanical compensation and optical compensation have been used in zoom lenses for decades. These techniques are used to maintain focus as the focal length of the zoom lens is changed. Use of a “floating element” in a lens is another case of mechanical compensation being used to optimize performance of a lens across a large range of distance. This technique changes the position of a lens element as the focus ring is moved. The Skyllaney Floating Lens Block (FLB) invention directly addresses change in focus as the aperture of the lens is changed. This technique changes the position of all or some of the parts of the optics as the aperture is changed.
The Sonnar optical design is especially affected by spherical aberration due to its compact, asymmetric design. The Skyllaney FLB invention originated to mitigate focus shift for the Bertele Sonnar. The FLB invention is applicable to a wide range of optical designs that have any degree of spherical aberration. The FLB invention can be used to mitigate focus shift due to spherical aberration on any type of lens—fixed focal length, floating element design, variable focal length lenses, and zoom lenses.
Author:
Brian Sweeney